Batch processes for the production of bis(hydroxyalkyl) terephthalate esters are known. For example, Doerr et al., U.S. Pat. No. 5,340,909 discloses a batch process for the manufacture of poly(1,3-propylene terephthalate), including a batch ester interchange reaction in which 1,3-propanediol is reacted with a lower dialkyl ester of terephthalic acid.
Continuous transesterification processes are known for the production of bis(2-hydroxyethyl) terephthalate. For example, Vodonik, U.S. Pat. No. 2,829,153, discloses a continuous process for the production of bis(2-hydroxyethyl) terephthalate and its low molecular weight oligomers from ethylene glycol and dimethylterephthalate.
It would be highly desirable to provide a continuous ester exchange process for the production of bis(3-hydroxypropyl) terephthalate and its low molecular weight oligomers from 1,3-propanediol and dimethylterephthalate. The present invention provides such a process.